Tips for identifying decorative stones -- Introduction

In this section

Decorative stones can have a wide variety of colours and
patterns, depending on their compositions and how they formed. When it comes to
identifying them, it helps to know a bit of geology. This short guide introduces
rocks, minerals and fossils, and points out some things to look out for when
identifying a polished stone. It uses examples from Corsi's collection to help
explain a number of useful geological terms.

You can search the database by entering some terms that
describe the geological features, colours and patterns that you see. Compare your
stone with the resulting gallery of images to see if any match. You can then read
about each stone sample to find out what Corsi wrote about it and what we know about
it today.

Remember… It is useful to have a torch when identifying stone in a
dark building. Take a hand lens or magnifying glass as well to see smaller details.
If a piece of stone is small enough, it can be examined under a binocular microscope.

If you are photographing a decorative stone to help identify it, take
a general view and then use the macro setting to take really sharp close-up pictures,
adding a small coin or measure to indicate scale. Avoid using flash if you can, as it
will reflect off the polished surface.

Time and place

People have been polishing stone for its beauty and usefulness
since prehistoric times. The ancient Romans sourced vast quantities of the
choicest varieties from all around the Empire to decorate the buildings of Rome.
Many Italian quarries opened to supply marble for the churches and palaces of the
Italian Renaissance and Baroque periods, while in late nineteenth and early
twentieth centuries, there was a renewed interest in the boldly marked, colourful
stones used by the ancient Romans. Concrete and glass may have dominated 1960s
architecture, but by the end of the 20th century, stone was again in demand,
particularly the neutral coloured limestones and granites.

The stone trade is now global, so blocks quarried in one country,
may be shipped to another for cutting and polishing, and yet another for final
use. Much of the polished stone we see today comes from the Middle East, Asia and
South America. When identifying a decorative stone, it is
important to think about where and when it was used. A stone's context can
offer some useful clues to its identity.

A few stones have been quarried fairly continuously for many
hundreds or even thousands of years, but most have been extracted only at certain
times in human history. So, an ancient artefact is unlikely to be made of a stone
that has only been quarried in more modern times.

On the other hand, a stone that was quarried thousands of years
ago does not always indicate an equally old artefact. People have been recycling stone since ancient times. For example, round disks
of marble sawn from ancient Roman pillars were often inlaid in the walls of later
buildings.

There have always been trends and fashions in
the use of stone. When a stone is found in an archaeological excavation, historic
building or in antique furniture, the age of the building or artefact can give
useful clues to the identity of the stone.

A bit of local knowledge can also be useful.
Some stones were only ever used near the place they were quarried, while others
have been transported all around the world. Even these may have been used in the
vicinity of the quarries for centuries before they were employed in the buildings
of other towns and cities. A geological survey, museum geologist, or university
geology department will often have specialist knowledge of the stones quarried in
their area.

A bit about names

All sorts of names are given to decorative stones. There are
Latin names used in antiquity, traditional names used by the Roman scalpellini of
Corsi's time, the classifications and trade names of the modern stone industry,
and finally, the scientific terms used by geologists.

The Latin names for decorative stones used by
ancient authors and recorded by Corsi in his catalogo
ragionato, are particularly of interest to archaeologists today.

Most of the contemporary names for stones used by Faustino Corsi
are the ones that were used by the scalpellini, the stone
cutters of Rome, for example rosso Verona and serpentina ranocchia. They often refer to the colour of the
stone and the place it was quarried. The scalpellini appended antico to indicate an ancient stone from the ruins of Rome.

In the stone trade today, any carbonate rock,
whether limestone or marble, is classified as a 'marble' if it takes a good
polish. Similarly, various different rocks composed of silicate minerals are
classified as 'granites'. Other classes of stone used by the trade include
'limestone' (if a carbonate rock doesn't pol/ish well), 'travertine', 'sandstone'
and 'slate'.

Many trade names are used for marketing
decorative stones. These sometimes indicate the place of quarrying for example Chiampo fiorito comes from the area of Chiampo in Italy. A
few bear a very similar name to a better known stone, for example Turkish Rosso lepanto, and the better known Italian Rosso Levanto. Usually), but usually they are fanciful names
aimed to appeal to customers, such as Blue pearl, or Sea green. Trade names need to be used with some
caution.

In geological nomenclature, decorative stones
are much more diverse. For example, Corsi's collection includes:

igneous rocks such as granites, gabbros, diorites,
andesites and phonolites;

sedimentary rocks such as limestones, travertines,
sandstones, breccias and conglomerates;

Minerals, rocks and fossils – introducing some geological terms

Minerals are the basic building blocks of the
inorganic natural world. Rocks are made up of minerals.

Each mineral has a particular chemical composition, and nearly all
are crystalline because the atoms are arranged in a regular geometric pattern.
Some rocks are made of just one mineral but most are composed of a mixture. It is
minerals that give rocks their colours and patterns, and many of their physical
properties. Even fossils, the lithified remains of ancient plants and animals, are
made up of minerals.

Rocks are classed as igneous, sedimentary or
metamorphic, depending on how they form.

Igneous rocks
crystallise out as molten rock, called
magma, cools. When rocks are eroded by the
effects of water, wind and ice, the fragments accumulate and are cemented
together to form sedimentary rocks. Sedimentary rocks also include muds precipitated at the bottom
of hot springs and deep oceans, and salt deposits formed by evaporation of
seas. When rocks are altered by the action of high temperatures and/or
pressures, they are said to be metamorphosed.
Metamorphic rocks form, for example, around igneous
intrusions and where continental plates are pushing together to throw up
mountain chains. You can also find out more about the different kinds of rock used
for decorative stone here.

Decorative rocks date from different times in the Earth's history.
The names and timescales for the different periods of geological time are shown in
the International Commission on Stratigraphy's International Stratigraphic Chart.

Fossils are the preserved remains of ancient plants
and animals, while trace fossils are the tracks,
burrows and other evidence of past life preserved in rocks.

Identifying stone - some things to look for

Colour

Is the stone the same colour all over
(monochrome), or is it a mixture of different
colours (polychrome)?

The purest marbles, limestones, travertines and alabasters are
white, but small amounts of mineral impurities can tint them beautiful
colours.

Hematite and goethite are responsible for most pinks, purples,
reds, oranges, and yellows. Iron-bearing silicate minerals including chlorite,
actinolite and serpentine give a green colour. Graphite makes a marble grey or
black, while bitumen can turn a limestone brown or black.

For some stones, the colour or palette of colours is a distinctive
feature that helps with identification.

Remember… when monochrome marbles and limestones lack other
distinctive features, they can be impossible to distinguish by eye. A thin section
of the stone under a petrological microscope reveals the size and shapes of the
grains and may give other clues to its geological history. Other procedures
analyse, for example, what trace elements are present, or the isotopic composition
of the stone. The results of these tests, when compared with that for
well-provenanced reference specimens, can indicate where a stone is from.

You may be able to see individual grains of coloured minerals in
metamorphic rocks and igneous rocks such as gneisses and granites.

Remember… polishing helps to bring out the colour. An
unpolished stone will have a duller colour, and black marbles look grey until they
are polished.

Crystals and grains

How big are the crystals or grains making up the stone? Are they
all the same size? What shape are they?

If the grains are big enough to see with the naked eye, it is a
coarse-grained rock. You will need a hand-lens to
see the individual grains in a medium-grained rock.
If the grains are too small to see, even with a hand lens, it is a fine-grained rock. Minerals in a fine-grained rock can
be identifiedby examining a very thin slice under a petrological microscope. A
natural glass such as obsidian, a rapidly cooled
volcanic rock, shows no grain structure at all.

Well formed crystals with straight sides are said to be euhedral. If they are not well formed and only just
show the crystal shape, they are subhedral. If they
are irregular grains and do not show the crystal shape at all, they are anhedral.

Are the crystals or grains all the same mineral, or are they a
mixture? Are they randomly mixed together? Does your rock have larger crystals in
a fine-grained groundmass?

If so, it is probably an igneous rock called a porphyry, and the big crystals are called phenocrysts. Metamorphic rocks can also have this
texture, in which case the large crystals are called porphyroblasts. The term megacrysts applies
to large crystals in a fine-grained groundmass, whether they are phenocrysts or
porphyroblasts.

Euhedral phenocrysts in a porphyry.

Anhedral porphyroblasts in a metamorphic rock

Looking at each mineral in the stone, can you identify it? Look at
its colour and lustre, test its hardness and look at the shapes of any
crystals.

Is your stone a soft calcareous rock
such as limestone, marble or travertine, or a hard siliceous rock such as a granite or gneiss?

While the colour of a mineral may depend on any impurities
present, its lustre, the way it reflects light, may be more
distinctive. Crystals of hematite and pyrite, for example, have a metallic lustre,
while the thin flakes of muscovite and biotite mica are particularly shiny and
reflective.

Each mineral has a particular hardness. Gypsum (calcium sulphate) is very soft and can be scratched
with a fingernail. Calcite and dolomite are harder but easily scratched with a
pen-knife. The feldspars are about the same hardness as a penknife blade. Quartz
is too hard to be scratched with a penknife blade. Remember, always test hardness
at an inconspicuous place on a sample.

You can also test for calcite by placing a small drip of dilute
acid (spirit vinegar or lemon juice may just work) and seeing if it fizzes, or
leaves a mark on the stone when it is wiped off, but BEWARE, a positive result
will damage a polished surface!

The geometric shapes of well-formed crystals among the grains in a
rock can give a big clue to the identity of the mineral.

Remember… polished stone is a natural material that can
show quite a lot of variation in colour and pattern, even among samples from the
same quarry. However, many decorative stones have particular characteristics that
mean they can be identified quite easily.

Examples of serpentino verde antico in Corsi's collection showing how it varies in
appearance.

Rocks within rocks

Is the stone composed of pieces or pebbles of rock cemented
together? Are the clasts (the pieces or pebbles of rock) rounded or angular?

Rocks with rounded clasts are called
conglomerates. The clasts are pebbles worn smooth
by the action of water or wind before being cemented together to form a new
rock.

A conglomerate

Rocks with angular clasts are called
breccias. They are formed where a rock has been broken up, for example by
an earthquake or flood, and quickly cemented together again.

A breccia

What kind of rocks are the clasts made of?

If they are too hard to scratch with a penknife, they are most
probably composed of quartz or hard silicate minerals. If they are softer and have
fossils in them, they are most probably limestones. If they are softer and have a
sugary texture, they are probably marble. Any kind of rock can form clasts in a
breccia or conglomerate.

Marble breccia

Limestone conglomerate

Do they all come from the same parent rock (monomict) or are they from a mixture of different
rocks (polymict)?

Monomict breccia

Polymict breccia

How big are the clasts? Are they well-sorted, meaning they are all about the same size or are they poorly-sorted, meaning they are a mixture of
different sizes?

Well-sorted breccia

Poorly sorted breccia

Do the clasts touch each other, or is each one completely
surrounded by the matrix?

Clast-supported breccia

Matrix supported breccia

Are the clast boundaries sharp and clear, or are they diffuse and
hard to distinguish? Does it look as though the clasts are stitched or sutured together?

Sharp clast boundaries

Diffuse clast
boundaries

sutured grain boundaries

What is the matrix like? Can you tell what it is made of?

The cement between the clasts is
called the matrix. You can look for many of the same
things in the matrix as you can for the clasts. For example, is it fine-, medium-,
or coarse-grained? Are the grains well-sorted or poorly sorted? Does it contain
any fossils or any interesting structures? Can you identify what minerals it is
made of?

Remember… when a stone has a large pattern, small pieces
can look very different even though they might be from the same slab.

The smaller
samples all come from this one slab of breccia measuring 2 x 1m.

Banding and layering

Does the stone have any linear or circular banding? Are the layers
composed of the same mineral or of different ones?

Linear banding is commonly seen when calcium carbonate
precipitates from supersaturated water, either to form stalagmites and stalactites
in caves, or to deposit a hot spring travertine. The coloured banding in many hot
spring deposits consists of iron oxides that were deposited by bacterial
action.

Linear banding in a travertine deposit

In other sedimentary rocks, linear coloured bands composed of
different minerals often show where layers of different sediments were deposited
on an ancient sea bed. When the rock has been metamorphosed, the layering may
be folded or contorted by the extreme
pressure.

Colour banding caused by layers containing different
minerals

Another kind of banding is shown byagate, a very fine-grained variety of quartz with bands coloured by
different impurities or minute gas bubbles. Veins of agate are commonly seen in
jaspers.

Agate banding in a vein in jasper.

Banding is often seen in igneous rocks where heavier crystals have
sunk in a molten lava, or where the flow of the lava has pulled crystals in a
particular direction.

Circular banding of different minerals is shown by orbicular granites and diorites, and occurs as the
molten magma cools, and crystals attach themselves in layers around central 'seed'
crystals.

Circular banding in an orbicular granite

Are the clasts aligned in a particular direction or deformed in
any way, for example sheared by metamorphic pressures?

When igneous rocks are metamorphosed, the pressure can align the
newly formed crystals into layers. Rocks with layers of aligned crystals are said
to be foliated.

Metamorphism shows in other ways, for example crystals, grains and
clasts can be stretched or sheared by metamorphic
processes. Rocks that show these textures are known as tectonites.

A foliated gneiss

Sheared marble breccia

Veins and stylolites

Are there any veins crossing the stone? Are they wide or narrow?
Are they randomly arranged or in a regular pattern? What mineral do they
contain?

Veins are cracks or
fractures in a rock that have been filled with minerals, often calcite
('spar') or quartz. They can be thick or thin, straight or curved, arranged
randomly or in a particular pattern.

Straight white veins filled with sparry calcite

Reticulated (net-like) array of veins in serpentinite

Are there any thin wiggly lines called stylolites? What color are
they?

Stylolites look like wiggly lines crossing the stone, rather
like the trace on a graph. They form when a rock is compressed under the weight of
more sediments and starts to recrystallise, part of an alteration process known as
diagenesis. The stylolites contain all the clay,
iron oxides and other impurities left behind, and so they are often a different
colour from the rest of the rock.

Are there any stylolites, and if so, what colour are they?

A stylolite in a limestone. There is a thin straight
vein running below it.

Fossils and other distinctive features

Does the rock contain fossils? How much of the rock is fossil
debris? Are the fossils whole or broken up? Can you tell what they are?

In decorative stones, fossils most commonly occur in limestones. A
limestone rich in fossils is known as lumachella.

A fossiliferous limestone

Are there any mossy, tree-like or shrub-like patterns?

These are called dendrites and are
delicate tree-like growths, usually of iron or manganese oxides. They often form
where chemical-rich solutions have penetrated slender fractures in a rock.

Dendrites in a fine-grained limestone

Dendrites in a travertine

Can you see angular areas of dark and light colour looking (with a
bit of imagination) like the ruins of buildings?

This is the 'paesina effect' named
after pietra paesina, an Italian ruin marble. It occurs
where microscopically thin fractures in the rock are sealed with calcite, and
restrict the spread of iron-staining by groundwater penetrating into the
stone.

The 'pietra paesina' effect

Does the stone have nodules giving a rounded lumpy appearance?

Nodules can form around fossils, by the action of bacteria on the
sea bed, and various other mechanism and are seen in some well-known decorative
stones.

Nodular limestone

Are there any empty or filled-up cavities in the stone?

Cavities and voids are not usually a good thing in a
decorative rock, and a porous structure that allows water to penetrate between the
grains makes the stone far less durable. However a few decorative stones do have
cavities and are still very durable. Sometimes the cavities are filled with a
mineral such as calcite ('spar') or quartz.

Empty cavities where plant material has rotted away are a common
feature of many travertines.

Cavities in travertine

Gas bubbles in volcanic rocks can also leave cavities called vesicles. If they subsequently fill up with crystals,
they are referred to as amygdales.

Amygdales in a volcanic rock, filled with white
crystals

Stromatactis is the name given to what appear to be
irregular shaped cavities filled up with layers of white and grey calcite. They
are found in certain limestones, usually of Devonian age. We are not certain how
they form. They are perhaps the relicts of sponges or corals, the product of
bacterial activity, or they are entirely inorganic sedimentary structures. It may
be that they result from a combination of these causes.

Stromatactis in a Devonian limestone

Question Checklist

Colour

Is the stone the same colour all
over (monochrome), or is it a mixture of different
colours (polychrome)?

Crystals and grains

How big are the crystals or grains making up the stone? Are
they all the same size? What shape are they?

Are the crystals or grains all the same mineral, or are they a
mixture? Are they randomly mixed together? Does your rock have larger crystals
in a fine-grained groundmass?

Looking at each mineral in the stone, can you identify it? Look
at its colour and lustre, test its hardness and look at the shapes of any
crystals.

Rocks within rocks

Is the stone composed of pieces or pebbles of rock cemented
together? Are the clasts (the pieces or pebbles of rock) rounded or
angular?

What kind of rocks are the clasts made of?

Do they all come from the same parent rock (monomict) or are they from a mixture of different
rocks (polymict)?

How big are the clasts? Are they well-sorted, meaning they are all about the same size or are they poorly-sorted, meaning they are a mixture of
different sizes?

Do the clasts touch each other, or is each one completely
surrounded by the matrix?

Are the clast boundaries sharp and clear, or are they diffuse
and hard to distinguish? Does it look as though the clasts are stitched or sutured together?

What is the matrix like? Can you tell what it is made of?

Banding and layering

Does the stone have any linear or circular banding? Are the
layers composed of the same mineral or of different ones?

Are the clasts aligned in a particular direction or deformed in
any way, for example sheared by metamorphic pressures?

Veins and stylolites

Are there any veins crossing the stone? Are they wide or
narrow? Are they randomly arranged or in a regular pattern? What mineral do
they contain?

Are there any thin wiggly lines called stylolites? What colour
are they?

Fossils and features

Does the rock contain fossils? How much of the rock is fossil
debris? Are the fossils whole or broken up? Can you tell what they are?

Are there any mossy, tree-like or shrub-like patterns?

Can you see angular areas of dark and light colour looking
(with a bit of imagination) like the ruins of buildings?

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